Than most likely the drive or external casing is bad. Try to take the drive out of the casing and connect it to a computer using the IDE or SATA port on the mainboard. If that does not work (if even the drive itself is not seen), it certainly is bad and ready for replacement.
1. To reset the Pram will reset the chip that looks for disks (among other resets). Reset the Pram (without the USB disk attached) to reset the ports on your Mac.
2. Are you sure the USB cable is good then? I ask because either the case, hard drive in the case or the USB cable will be the problem.